High electron mobility transistor and method for forming the same

A method for forming a high electron mobility transistor, comprising:

• providing a substrate;
• forming a mesa structure on the substrate, wherein the mesa structure has a channel layer and a barrier layer disposed on the channel layer;
• forming a semiconductor gate layer on the barrier layer of the mesa structure;
• forming a passivation layer covering the substrate and the mesa structure and the semiconductor gate layer;
• forming at least a first opening at a side of the semiconductor gate layer, wherein the first opening penetrates through the passivation layer and extends into part of the barrier layer, wherein a bottom surface of the first opening is lower than an upper surface of the barrier layer and higher than a bottom surface of the barrier layer;
• forming a plurality of second openings, wherein the plurality of second openings are connected to a bottom surface of the first opening and penetrate through the barrier layer and a portion of the channel layer;
• forming a metal layer in the first opening and the plurality of second openings thereby forming a contact structure;
• and
• forming a gate metal layer through the passivation layer to directly contact a top surface of the semiconductor gate layer.